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To care for the weak, the unfit, and the cripple is usually considered
an extremely highly developed altruism in our society. As our society
progresses, more and more provisions are made for the unfit.
In nature the unfit usually is soon weeded out. If an animal is
unable to feed itself it is doomed; or if it is less successful than
its fellows it has less chance of leaving progeny. That is natural
selection.
Hence on both counts it comes as a surprise to find two
well-authenticated cases of crippled birds, unable to search for food
for themselves, surviving for long periods.
The first is T. R. Peale's record in 1848 of a brown booby on Enderby
Island in the Pacific. An adult bird whose plumage indicated it was
several years old was found on the island, and it had only one wing,
the other having been lost by some accident and the wound completely
healed. The bird was unable to go to sea and get its own food, and was
being fed by its fellows.
The second was a frigate bird, found on the Revillagigedo Islands,
reported by A. W. Anthony in 1898. This bird, too, was fat, and had one
wing withered and useless, evidently from hatching. It had never flown.
Frigate birds are masters of the air that snatch their food on the wing
from the surface of the water, and a flightless frigate bird would be
as badly off as a flightless swallow. The cripple had been fed all its
life by its neighbors.
At first the uncritical might think, What altruism, what charity, for
the healthy to feed these two cripples. But an explanation involving
less advanced principles, principles more in keeping with what we know
about bird behavior, is possible. Remember that young birds that are
unable to begin feeding themselves at the proper time may continue to
beg for food, and be dependent for a long time, as I have shown with
young shrikes under the chapter, "Conditioning in Birds." Remember that
a young bird begging for food may be fed by adults, not its parents,
and even by other young birds (shown in "Bird Helpers at Nesting
Time"); and we have the clue.
The cripples, hungry, begged for food; the healthy birds responded by
feeding, as they might do to other begging young, and owing to the
unusual circumstances both were continued.
These certainly are cases where the unfit survived. Natural selection
has not operated. But such cases are rare exceptions.